The long-term goal of this research is to gain a quantitative understanding of the regulation of the eukaryotic cell cycle, one of the most fundamental and complex biological processes. The specific goal of this proposal is to construct, analyze and validate mathematical models for several key cell cycle checkpoints and switches as well as for the entire cell cycle network in the budding yeast Saccharomyces cerevisiae, through a close coupling between mathematical modeling and quantitative experimentation. Cell cycle checkpoints and switches play essential roles in ensuring precise and robust execution of the cell cycle machinery. Defects in them, e.g. due to genetic perturbations, can lead to inappropriate cell proliferation or errors in chromosome segregation, which are commonly associated with tumorigenesis. This work will contribute to a deeper, quantitative and systems level understanding of the yeast cell cycle regulation, the studies of which have profoundly impacted on our knowledge about cell cycle control in higher organisms and on the mechanisms of cancer. Concepts and methods from dynamical systems theory will be applied here to analyze and comprehend this complex system. Quantitative single-cell assays using microfluidic devices will be set up to generate data for the mathematical model and to test the model predictions. The specific aims are: (1) Global computational analyses of the yeast cell cycle network - Systematic computational analyses on the yeast cell cycle network will be carried out to investigate the global dynamic properties and the structural stability of the system to identify what kinds of perturbations the system is robust to and what is not. (2) Quantitative study of the G1 checkpoint as a fixed point - Computational models and quantitative experiments will be used together to investigate the stability of the G1 arrest and the network perturbations that can increase or decrease this stability. The hypothesis that the checkpoint is a dynamical system's fixed point will be tested. (3) Quantitative study of the G1/S switch - Computational models and quantitative experiments will be used together to investigate the switch-like behavior in S-phase entry. The role of the circuit topology in ensuring a robust switching behavior will be studied. (4) Quantitative study of the spindle assembly checkpoint and the M/A switch - Computational modeling and quantitative experiments will be used together to investigate the stability of the checkpoint arrest and the switching dynamics, focusing on the respective and synergistic roles of the multiple feedback loops.